In high speed serial data communications links, there are losses in signal integrity as data is transmitted and received through a less than ideal channel. For example, a data signal traveling through a cable must eventually pass onto a line card via an SMA connector, onto an fr4 or getek trace, through a packaging and finally onto a SERDES (serializer/deserializer) transceiver (i.e., a transmitter/receiver pair) that converts data received in serial format to data in parallel format. Another example is the propagation of a signal from one line card to another, first passing through a Tyco or Teradyne connector, onto a getek backplane (analogous to a “mother” board) and back again. As the signal propagates through these media, it experiences losses through non-ideal transmission line effects as well as lumped parasitic elements at the interfaces that act to attenuate high frequency components and distort the signal. The result is an increase in jitter, which closes the periodic valid data window, known as the data eye. The data eye provides a measure of the quality of the channel and the quality of the SERDES. The Bit Error Rate (BER) increases as the data eye closes.
High speed serial data communications links, in which such losses in signal integrity occur, have been arranged to counteract the effect of high frequency attenuation and improve the signal integrity as data is transmitted and received through a less than ideal channel. Generally, the data transmitter has circuitry that amplifies the high frequency content of the data being transferred into the channel more than the amplification of the low frequency content of the data being transferred into the channel. The degree of amplification of each frequency component of the data signals is controllable.
In practice, several different lengths of cable and/or lengths of backplane are characterized to determine the optimum settings for the amount of correction performed by the transceiver to result in minimum jitter. Customers are given information about these settings. The SERDES transceiver can be placed into the system and the transmitted data eye, at a given point within the system, optimized by manual manipulation of the control and observation of the data eye. These settings then are applied to all of the same SERDES units for this application.
The techniques used in the past and described generally above involve an “equalization” function that can be characterized as “preset” equalization. Because preset equalization requires the setting of each unit, preset equalization is considered less than desirable in certain applications.